Currently, the most powerful Sekirei MIB has is Karasuba. She's the only one who's stuck around after the first Disciplinary Squad disbanded. So clearly, her role in the story is going to be that of an end-boss of sorts. Normally it might seem like her fellow members such as Kazehana or Miya might be the ones to take her on, but no doubt they'll conveniently be occupied elsewhere at the time, due to the build-up gabriel blessing has spaced throughout the story between the two.
The first half of the fight is going to feature Karasuba kicking Shirou's ass. She's not quite as powerful as Miya, obviously, but Shirou has noted that even amongst his Sekirei his abilities are probably inadequate comparatively speaking. He's probably going to surprise her a bit with his tracing, but ultimately it's not going to be enough.
Of course, gabriel blessing has made clear that while Shirou has not YET mastered Unlimited Blade Works, it's one of his goals. No doubt by the time of the fight, he'll be in a position where it's time to use it. This, very obviously, will be where the battle turns. Karasuba will be overwhelmed by what's going on, and Shirou will use the fact he can now spam some of his more powerful Noble Phantasms to turn the tide. The fight will probably end with Shirou disarming Karasuba and having a blade to her throat.
Obviously that's the end of the fight, but regardless of the fact she found an Ashikabi, Karasuba is going to be on Shirou like white on rice after his display. The harem, after all, must be fed.
- Half-Jossed. While Karasuba did join the Harem, the final fight was against Minaka.
Include some of the above information but also factor in Rin Tohsaka and Saber's appearances. This fight is more unpredictable with more opponents, but if they all gang up on Shirou the possibility for his defeat increases drastically. At the same time, Karasuba has no clue on how to approach Rin and Saber because she never met them before, and has no clue that if Shirou loses that may be the last time she sees him. Much more unpredictable than the above fight.
- Too bad the idea is now Jossed.